1. Field of the Invention
The present invention relates to an emission drive circuit and its drive control method and a display unit and its display drive method. Particularly, the present invention relates to a light emission drive circuit that can apply a current control type (or a current drive type) of light emission element emitting light at a predetermined luminance gradation sequence by supplying a current in accordance with the display data to plural display panels (pixel arrays) and its drive control method, and a display unit provided to each display pixel and its display drive method.
2. Description of the Related Art
In recent years, as a monitor and a display of a personal computer and a video system, a display device in place of a conventional display unit applying a conventional cathode-ray tube (CRT) has been widely used. Particularly, a liquid crystal display (LCD) has been rapidly widespread because it can be made thinner, lighter, spacious, and lower-power consumption or the like, in comparison with the conventional display. In addition, a relatively small liquid crystal display has been also widely applied as a display device that has been remarkably widespread in recent years such as a cellular phone, a digital camera, and a personal digital assistance (PDA).
As a next-generation display device (display) following such a liquid crystal display, a full-scale commercial viability and diffusion of a light emission element type of display device, in which an organic electro luminescence (hereinafter, abbreviated as “an organic EL element”) and an inorganic electro luminescence (hereinafter, abbreviated as “an inorganic EL element”) or a light emission element (a self-luminous type of a display pixel) such as a light emission diode (LED) are arranged in a matrix, has been expected.
Particularly, in comparison with the above-described liquid crystal display, the light emission element type of display applying an active matrix drive system has a higher display response speed, no viewing angle dependency, a high luminance, a high contrast, and a high resolution of a display image quality or the like. Further, the light emission element type of display does not need a back light as the liquid crystal display. Therefore, the light emission element type of display has a very superior characteristic such that it can be made further thinner and lighter and a low-power consumption is possible.
In such a light emission element type of display, various drive control mechanisms and control methods for controlling the operation of the light emission element (the light emission state) are suggested. For example, as described in Jpn. Pat. Appln. KOKAI Publication No. 8-330600, there has been known a configuration including a drive circuit provided with a plurality of switching elements for light-emission-drive controlling the light emission element (hereinafter, abbreviated as “a light emission drive circuit”) for each display pixel to compose a display panel in addition to the above-described light emission element.
FIG. 22 is a schematic block diagram showing a substantial part of a voltage control active matrix light emission element type of display according to the prior art. FIG. 23 is an equivalent circuit diagram showing a constitutional example of a display pixel (a light emission drive circuit and a light emission element) that can be applied to a light emission element type of display according to the prior art. Here, in FIG. 23, the circuit configuration provided with an organic EL element as the light emission element is shown.
An active matrix type of organic EL display unit described in Jpn. Pat. Appln. KOKAI Publication No. 8-330600, as roughly illustrated in FIG. 22, is configured so as to comprise: a display panel 110P in which a plurality of display pixels EMp are arranged in a matrix in the vicinity of each intersecting point of a plurality of scan lines (a selection line; a signal line in a Y direction) SLp arranged in row and column directions respectively and a data line (a signal line; a signal line in a X direction) DLp; a scan driver (a Y directional peripheral drive circuit) 120P connected to each scan line SLp; and a data driver (a X directional peripheral drive circuit) 130P connected to each data line DL.
Each of display pixels EMp, as shown in FIG. 23, is configured so as to have: a light emission drive circuit DCp including a thin film transistor (TFT) Tr 111 in which a gate terminal is connected to the scan line SLp and a source terminal and a drain terminal are connected to the data line DL and a contact point N111, respectively, and a thin film transistor Tr 112 in which the gate terminal is connected to the contact point N111 and a predetermined power source voltage Vdd is applied to the source terminal; and an organic EL element (a current control type of a light emission element) OEL in which an anode terminal is connected to the drain terminal of a thin film transistor Tr 112 of the light emission drive circuit DCp and a ground potential Vgnd that is a lower potential than the power source voltage Vdd is applied to a cathode terminal. Here, in FIG. 23, reference symbol Cp denotes a condenser to be formed between the gate sources of the thin film transistor Tr 112.
In the display unit including the display panel 110P configured by the display pixel EMp having such a structure, first, an on-level scan signal voltage Ssel is sequentially applied from the scan driver 120P to each scan line SLp, whereby the thin film transistor Tr 111 of the display pixel EMp (the light emission drive circuit DCp) for each row is turned on and the display pixel EMp is set at a selection state.
By applying a gradation sequence signal voltage Vpix in accordance with the display data to the data line DLp of each row by a data driver 130P in synchronization with this selection timing, a potential corresponding to the gradation sequence signal voltage Vpix is applied to the contact point N111 (namely, the gate terminal of the thin film transistor Tr 112) via the thin film transistor Tr 111 of each display pixel EMp (the light emission drive circuit DCp).
Thereby, the film transistor Tr 112 is turned on in a conducting state in accordance with the potential of the connect point N111 (namely, a conducting state in accordance with the gradation sequence signal voltage Vpix). Then, a predetermined light emission drive current is supplied from the power source voltage Vdd to the ground potential Vgnd via the thin film transistor Tr 112 and the organic EL element OEL, and the organic EL element OEL performs the light emission operation at a luminance gradation sequence in accordance with the display data (the gradation sequence signal voltage Vpix).
Next, by applying an off-level scan signal voltage Ssel to the scan line SLp from the scan driver 120P, the thin film transistor Tr 111 of the display pixel EMp for each row is turned off, the display pixel EMp is set at a no-selection state, and the data line DLp and the light emission drive circuit DCp are electrically shielded. In this case, when the potential applied to the gate terminal (the contact point N111) of the thin film transistor Tr 112 is kept in the condenser Cp, a predetermined potential is applied between the gate sources of this thin film transistor Tr 112, and this results in that the thin film transistor Tr 112 is kept in the on state.
Accordingly, as same as the light emission operation in the above-described selection state, a predetermined light emission drive current is supplied from the power source voltage Vdd to the organic EL element OEL via the thin film transistor Tr 112 and the light emission operation continues. The light emission operation is controlled so as to be continued, for example, on one frame till the gradation sequence signal voltage Vpix corresponding to the next display data is applied (written) in the display pixel EMp of each row.
Such a voltage drive control method is called as a voltage gradation sequence designation system (or a voltage gradation sequence designation driving) because the current value of the light emission drive current to be supplied to the organic EL element OEL is controlled by controlling the voltage value of the voltage (the gradation sequence signal voltage Vpix) to be applied to each display pixel EMp (specifically, the gate terminal of the thin film transistor Tr 112 of the light emission drive circuit DCp) so as to perform the light emission operation at a predetermined luminance gradation sequence.
The display unit in which the light emission drive circuit corresponding to the voltage gradation sequence designation system is provided to each display pixel involves the following problem.
In the light emission drive circuit DCp as shown in FIG. 23, a current path is connected to the organic EL element OEL in series and the operation property (particularly, the threshold voltage value property) of the thin film transistor Tr 112 for the light driving to supply the light drive current corresponding to the display data (the gradation sequence signal voltage) is changed (temporarily changed) depending on the usage time or the like. In such a case, the current value of the light emission drive current (the current between the source and the drain) flowing between the source and the drain at the predetermined gate voltage (the potential of the contact point 111) is varied (for example, decreased). For this reason, it becomes difficult to stably realize the light emission operation at the appropriate luminance gradation sequence in accordance with the display data for a long time.
In addition, in the case where the element properties (the threshold voltage property) of the thin film transistors Tr 111 and 112 within the display panel 110P are variable for each light emission drive circuit DCp or in the case where the element properties of the thin film transistors Tr 111 and 112 are variable for each display panel 110P depending on a production lot, the above-described variation of the current value of the light emission drive current becomes large in the light emission drive circuit of the voltage gradation sequence designation system. For this reason, the appropriate gradation sequence control cannot be carried out and the display image quality is lowered.